1. Field of the Invention
This invention relates to an ignition timing control system for internal combustion engines, and more particularly to an ignition timing control system for an internal combustion engine having an adsorber for adsorbing unburnt gas and an exhaust gas-purifying device equipped with a heater, which are arranged in series in an exhaust passage of the engine.
2. Prior Art
Conventionally, a technique has been proposed by the present assignee e.g. by Japanese Provisional Patent Publications (Kokai) Nos. 61-205376 and 61-205377, in which under a predetermined operating condition of an internal combustion engine immediately after the start of the engine, ignition timing is retarded relative to an ordinary ignition timing to elevate exhaust gas temperature, thereby promptly warming up a catalytic converter (three-way catalyst) arranged in an exhaust passage of the engine to activate the same.
Further, in order to control emission of unburnt components (HC (hydrocarbon), etc.) into the atmosphere when an internal combustion engine is started in a cold state, it has been proposed, e.g. by Japanese Provisional Utility Model Publication (Kokai) No. 4-105925, to provide an adsorber for adsorbing unburnt components contained in exhaust gases, and a catalytic converter equipped with a heater (hereinafter referred to as "the heater-equipped catalyst"), which are arranged in an exhaust system of the engine.
The adsorber has a characteristic that it releases unburnt components adsorbed therein at a temperature higher than a predetermined temperature (approximately 200.degree. C.). Therefore, if a three-way catalyst (main catalytic converter) is arranged downstream of the adsorber and the heater-equipped catalytic converter, it is required to warm up the heater-equipped catalyst within a short time, so as to prevent activation of the heater-equipped catalyst or the main catalytic converter from being delayed to a time later than the time of release of unburnt components from the adsorber.
One way to meet this requirement would be to start ignition timing retardation at the start of the engine to elevate the exhaust gas temperature to a higher level, whereby it is possible to accelerate activation of the heater-equipped catalyst. On the other hand, the adsorber is originally intended to temporarily adsorb unburnt components when the heater-equipped catalyst or the main catalytic converter has not been activated, and release adsorbed unburnt components therefrom when the main catalytic converter has become activated, to thereby allow most of the unburnt components to be purified by the heater-equipped catalyst and the main catalytic converter.
If the ignition timing retardation is started simultaneously with the start of the engine to promptly warm up the heater-equipped catalyst and the main catalytic converter, the temperature of the adsorber arranged upstream of them is also elevated promptly, so that the temperature of the adsorber becomes higher than the predetermined temperature (approximately 200.degree. C.) to release unburnt components adsorbed therein before the heater-equipped catalyst and the main catalytic converter become activated. As a result, the unburnt components are undesirably emitted into the atmosphere without being purified by the heater-equipped catalyst and the main catalytic converter. In short, to control the emission of unburnt components into the atmosphere, it is necessary to prevent unburnt components from being released from the adsorber before the heater-equipped catalyst and the main catalytic converter become activated.
Further, if a countermeasure is taken simply by reducing an amount of ignition timing retardation or by inhibiting the ignition timing retardation per se, it takes a longer time period to activate the catalysts, which would then require an increased electrical capacity of the heater-equipped catalyst or a longer time period for energization of the heater to accelerate activation of the catalysts, resulting in a new problem of an increased consumption of electric power.